Update graphics/ImageMagick6 to 6.9.9.20.

6.9.9.20
- Return expected results for a percent 0 -chop option argument
- Tweaks to OpenMP support within ImageMagick.

6.9.9.19
- Correct handling of GIF transparency

6.9.9.18
- Resetting the magick_list_initialized boolean when needed

6.9.9.17
- Fix broken build

6.9.9.16
- Fixed numerous memory leaks
- Support URW-base35 fonts.

6.9.9.15
- Fixed numerous memory leaks

6.9.9.14
- Fixed numerous memory leaks
- Stop poential leaks in the JNG decoder
- Maximum valid hour is 23, not 24, in the PNG tIME chunk, and maximum
  valid minute is 59, not 60.

6.9.9.13
- Use signed integer arithmetic to calculate timezone corrections

(fhajny)